Sweep oscillator

ABSTRACT

A sweep oscillator used to generate a sawtooth wave that can be amplified to serve for deflecting the electron beam of a cathoderay tube in synchronism with any of input pulses, where the different slopes of the sawtooth wave are successively switched automatically until any of the input pulses is not received within a predetermined first interval starting in synchronism with the start of the sawtooth wave and, when the number of the input pulses received within a predetermined second interval included within the duration of the sawtooth wave and longer than the first interval is less than the predetermined number, a sawtooth wave having the gentlest slope is generated.

United States Patent s41 swear OSCILLATOR 3Claims,l7l)rawingfigs. [52]Us. 331/143, 328/i85,33l/153 [5!] Int. H03k4/08 [50] FieldotSearch331/143,

sweep GATE [56] References Cited UNITED STATES PATENTS 3,493,961 2/1970Hansen Primary Examiner-John Kominski Attorneys-Robert E. Burns andEmmanuel .l. Lobato ABSTRACT: A sweep oscillator used to generate asawtooth wave that can be amplified to serve for deflecting the electronbeam of a cathode-ray tube in synchronism with any of input pulses,where thedifferent slopes of the sawtooth wave are successively switchedautomatically until any of the input pulses is not received within apredetermined first interval starting in synchronism with the start ofthe sawtooth wave and, when the number of the input pulses receivedwithin a predetermined second interval included within the duration ofthe sawtooth wave and longer than the first interval is less than thepredetermined number, a sawtooth wave having the gentlest slope isgenerated.

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swans oscutn'ron This invention relates to a sweep oscillator used togenerate a sawtooth wave that can be amplified to serve for deflectingthe electron beam of a cathode-ray tube in synchronism with any of inputpulses.

in conventional sweep oscillators of this type, the number of cycles tobe displayed on a cathode-ray tube in preset in case of observation ofthe waveform of an input signal, and the sweep period of the sawtoothwave generated is automatically switched in response to the startcontrol by a manual switch so that the input signal of the preset numberof cycles can be observed on the cathode-ray tube.

However, the conventional sweep oscillators have disadvantages such thatthe automatic switch operation of the sweep period of the sawtooth waveis not started without control of the manual switch, and thatif thefrequency of the input signal alters from a high frequency to a lowerfrequency the sweep period of the sawtooth wave is not automaticallyswitched without another control of the manual switch. in other words,the sweep period of the sawtooth wave is not switched automatically inaccordance with the change of the frequency of the input signal.

An object of this invention is to provide an automatic sweep oscillatoreliminatable of the above-mentioned defects of the conventional ones,switchable successively of the period of the sawtooth wave towards theshorter period if the frequency of the input signal alters from lower tohigher, and resettable automatically of the longest period of thesawtooth wave if the frequency of the input signal alters from higherto-lower.

Accordingly, the automatic sweep oscillator of this invention has thefollowing merits over the conventional ones:

1. Manual control is not necessary even if the frequency of the inputsignal alters.

2. Since the period of the sawtooth wave is automatically switched inaccordance with the frequency of the input signal, steadysynchronization of the sawtooth wave with the frequency of the inputsignal is always obtained.

The principle of this invention will be better understood from thefollowing more detailed discussion taken in conjunction with theaccompanying drawings, in which the same or equivalent parts aredesignated by the same reference numerals, characters and symbols, andin which:

Fit l is a block diagram for describing the principle of construction ofthis invention;

H68. 2, 3, 4, 5, 6 and 7 are block diagrams each for illustrating anembodiment of this invention;

FlGS. d, 9, it), ll, 12 and 13 are time charts explanatory of theoperations of the embodiments shown in FIGS. 2, 3, 4i, 5, 6 and 7respectively;

HG. l4 is a block diagram for illustrating in details an embodiment ofthis invention;

H08. l5, l6 and 17 are time charts explanatory of the operation of theembodiment shown in HQ 14; and

HG. 16 shows time charts explanatory of the principle of determiningreference levels used in the embodiment shown in FIG. l4.

With reference to P16. 1, the principle of construction of thisinvention will first be described. This automatic sweep oscillator ofthis invention for generating a sawtooth wave synchronized with any ofinput pulses applied from an input terminal Vl comprises: (1) a sawtoothwave generator I for generating a sawtooth wave synchronized with one ofa plurality of predetermined different slopes, (2) a hold circuit llgenerating an output when the sawtooth wave reaches a predeterminedlevel, (3) a control circuit ill for determining the duration of thesawtooth wave by starting the sawtooth wave in synchronism with one ofthe input pulses and by terminating the sawtooth wave in synchronismwith the output of the hold circuit ll, (4) an auto selection circuit IVfor switching successively the different slopes of the sawtooth wavefrom a gentle one to a sharp one and for stopping the switching when anyof the input pulses is not received within a predetermined firstinterval starting in synchronism with the start of the sawtooth wave,and (5) a reset circuit V for controlling the automatic selectioncircuit lV so that the sawtooth wave generator l generates a sawtoothwave having the gentlest slope when the number of input pulses receivedwithin a predetermined second interval included within the duration ofthe sawtooth wave and longer than the first interval is less than thepredetermined number. The sawtooth wave generated is obtained at anoutput terminal Vll. Actual embodiments of this invention will bedescribed below.

in an embodiment of this invention shown in FIG. 2 a sweep gate 2produces a sweep gate signal w in response to one of input pulses w.applied from an input terminal 1. A sawtooth wave generator 4 generatinga sawtooth wave w in synchronism with the sweep gate signal W A holdcircuit 21 for closing the sweep gate 2 when the sawtooth wave w reachesa voltage (determined so as to be sufficient to fullsweep the horizontalscanning of a cathode-ray tube in case of applying this invention to anoscilloscope using the cathoderay tube) and for holding the closed stateof the gate 2 until restoration of the level of the sawtooth wave w;, tothe original level. A first gate 6 produces a gate signal w, started insynchronism with the start of the sweep gate signal W and terminatedwhen the sawtooth wave w reaches a first predetermined level. An ANDgate 8 passes the input pulses w only when the gate signal w is appliedfrom the first gate 6. A monostable circuit 9 is triggered by the outputof the AND gate 8 and produces an output pulse having a width determinedby the time constant of this circuit 9. A counter it) counts the numberof output pulses from the monostable circuit 9. Predetermined differentslopes of the sawtooth wave W3 are switched in accordance with thecounting state of the counter 10. A second gate 12 is opened insynchronism with the termination of the gate signal w., and closed whenthe sawtooth wave w reaches a predetermined second level. An inhibitgate 13 is opened in case of no output of a l/N frequency divider 14(described below) and passes the input pulses only in the opened periodof the second gate 12. The l/N frequency divider 1d divides by l/N therepetition frequency of the pulses passed through the inhibit gate 13.This division ratio MN is determined in consideration of the number ofcycles to be displayed on the cathode-ray tube. A comparator w generatesan output pulse w when the level of the sawtooth wave reaches apredetermined threshold level. An inhibit gate 16 inhibits or passes theoutput pulse w of the comparator 19 in response to generation ornongeneration of the output pulse w from the l/N frequency dividerrespectively. A monostable circuit 17 is triggered by the output pulseW9 of the inhibit gate 16 and generates a reset pulse the duration ofwhich is determined in accordance with the time constant of thismonostable circuit 17. This reset pulse is applied to the counter 10 toreset the counting state of the counter 10 so as to make the sawtoothwave generator t generate the sawtooth wave w having the gentlest slope.

The operation of this embodiment will be described with reference to H6.7.

At first, the operation of successive automatic selection of differentsweep periods is described. It is assumed that the input signal is apulse train (w in FIG. 8) timed with successive cycles of a signal to besynchronized with the sawtooth wave W3. The state of the gate signal Wis revemd in response to one of pulses of the pulse train w,. Thesawtooth wave generator 4 starts the generation of the sawtooth wave w,,in response to the reverse of the state of the gate signal W2. The firstgate 6 is opened at the same time as shown by the gate signal W4 andclosed at an instant when the level of the sawtooth wave W3 reaches apredetermined level at a point A. W pulses of the input pulse train ware received within the duration of this gate signal W4, these pulsespass through the AND gate 8 and trigger the monostable circuit 9. Thenumber of the output pulses of the monostable circuit 9 is counted bythe counter 10. Accordingly, the counting state of the counter ill) iscounted up and the predetermined different slopes of the sawtooth waveW3 corresponding to the successive counting states of the counter 10 aresuccessively switched at the sawtooth generator 4 in accordance with thesuccessive counting up of the counting state of the counter 10. Thisswitching operation of the slope of the sawtooth wave w continues untilno pulse is received within the duration of the gate signal w.,.

The reset operation of this embodiment to an original state where thesawtooth wave generator 4 generates the sawtooth wave having thegentlest slope will be described below. The second gate 12 generates thegate signal W5 started in timed with the termination of the gate signalw and terminated when the level of the sawtooth wave w reaches apredetennined threshold level at a point B. The input pulses (w, appliedwithin the duration of this gate signal w passes through the inhibitgate 13 unless this inhibit gate 13 is closed by the output of the 1/Nfrequency divider as mentioned below. If it is as sumed that the l/Nfrequency divider 14 is designed as a 1/3 frequency divider, the stateof the output signal w of this frequency divider 14 is reversed by thethird of pulses (w,) passed through the inhibit gate 13 as illustratedin FIG. 8. At the same time, the inhibit gate 13 is closed by thisoutput signal w, so that any of the input pulses (w,) cannot passthrough the inhibit gate 13. Moreover, the inhibit gate 16 is closed bythe output signal w so that the output pulse w,, of the comparator 19generated when the level of the sawtooth wave reaches a predeterminedlevel at a point C cannot pass through the inhibit gate 16. However, ifthe state of the frequency divider 14 is not reversed within theduration of the gate signal w, the output pulse w, of the comparator 19passes through the inhibit gate 16 opened and triggers the monostablecircuit 17. This condition is obtained in the case where the number ofpulses (w,) included within the duration of the gate signal W5 is lessthan the number (N) predeternrined. The output pulse of the monostablecircuit 17 resets the counter to its original counting statecorresponding to the gentlest slope of the sawtooth wave w As mentionedabove, the successive selection of different sweep periods (i.e.;different slopes) of the sawtooth wave w and the resetting to thegentlest slope of the sawtooth wave W3 can be automatically performed bythe embodiment of this invention shown in FIG. 2. In other words, if thesawtooth wave w if applied as the horizontal scanning wave (sweepsignal) of an oscilloscope using a cathode-ray tube, the period of thesawtooth wave W is successively switched in an automatic manner fromlonger to shorter until the number of cycles of a signal displayed onthe oscilloscope becomes equal to a predetermined number N determined bythe frequency-division ratio 1/N of the frequency divider 14. Moreover,this synchronized condition will be satisfied by the same operation asmentioned above if the frequency of the displayed signal changes fromlower to higher. However, the frequency of the displayed signal changesfrom higher to lower after the abovementioned synchronization, thesucceeding synchronization will be obtained by perfonning theabove-mentioned switching operation after the resetting of the period ofthe sawtooth wave w to the shortest period in accordance with the forementioned resetting operation.

With reference to FIGS. 3 and 9, another embodiment of this inventionwill be described. In this embodiment, circuits 2, 4, 6, 8, 9, 10, 12,13, l4, 16, 17 and 21 are the same as those of FIG. 2 respectively. Aditferentiator 19 generates a pulse w timed with the rise time (start)of the gate signal w applied from the sweep gate 2. A delay circuit 20delays the output signal w, of the llN frequency divider by a time A T.In this embodiment, the operation in successively selecting thedifferent slopes of the sawtooth wave W3 is the same as the operation inthe embodiment shown in FIG. 2. However, the resetting operation to theshortest period of the sawtooth wave w, is different from the resettingoperation of the embodiment shown in FIG. 2. The delay time A T of thedelay circuit 20 is determined so as to be sufficiently longer than theduration of the output pulse w of the differentiator 19 so that theoutput pulse w cannot pass through the inhibit gate 16 unless the outputw, of the 1/N frequency divider 14 is generated. As the result of thisconstruction, the output pulse w, can be passed through the inhibit gate16 only when the above mentioned synchronization condition is performed,so that the output pulse W9 triggers the monostable circuit 17 so as toreset the circuits 4, 9, 10 and 14.

FIG. 4 shows another embodiment of this invention, in which all ofcircuits except the second gate 12 are the same as those shown in FIG. 2respectively. While one of the inputs of the second gate 12 is connectedto a line 7 of the output of the first gate 6 in FIG. 2, one of theinputs of the second gate 12 of this embodiment is connected to a line 3of the output of the sweep gate 2. Operational waves W W w and W4 ofthis embodiment are, as shown in FIG. 10, the same as the waves w,, W Wand w shown in FIG. 8 respectively. While the start of the second gate12 is timed with the termination time of the gate signal W; in theembodiment shown in FIG. 1, the start of this second gate 12 in thisembodiment is timed with the start of the gate signal W2. Thetermination of the gate signal W5 obtained from the second gate 12 isdelayed by an appropriate time from the termination time of the gatesignal w, obtained from the first gate 6. These termination times of thegate signals W4 and W5 are determined in the gates 6 and 12 as shown bypoints A and B in FIG. 10 respectively. Threshold levels determining thepoints A and B are concerned in the number of pulses of the input pulsetrain w, included within the duration of the sawtooth wave W3.

FIG. 5 shows another embodiment of this invention which comprises theautomatic selection function (i.e.; circuits 2, 4, 21, 6, 8, 9 and 10)of the embodiment shown in FIG. 3 and the resetting function (i.e.;circuits 12, 13,14, 16, 17, 19 and 20) of the embodiment shown in FIG.4. Operative waves of respective parts of this embodiment are shown inFIG. 11. Details of the operation of this embodiment are omitted sincethis operations will be readily understood with reference to theoperations of the forementioned embodiments.

FIG. 6 is another embodiment of this invention, in which a bistablecircuit 12a is employed instead of the second gate 12. The state of thisbistable circuit 12a is set in response to the termination of the gatesignal W4 obtained from the first gate 6 and reset in response to thetermination of the gate signal w obtained from the sweep gate 2. Othercircuits are the same as those of the embodiment shown in FIG. 2.Operational waves of respective parts of this embodiment are shown inFIG. 12. Waves w,, w, w;, and w, are the same as those shown in FIG. 8respectively. The wave w, of the output of the bistable circuit startsin response to the termination of the gate signal w. obtained from thefirst gate 6 and terminates in response to the termination of the gatesignal w obtained from the sweep gate 2. Other waves w to W9 are thesame as those shown in FIG. 8 respectively.

FIG. 7 shows a modification of the embodiment shown in FIG. 6, in whichthe differentiator 19 and the delay circuit 20 described in theembodiments of FIGS. 3 and 5 are employed instead of the comparator 19.Operational waves of respective parts of this embodiment are shown inFIG. 13. Waves w,, W W W4, w,,, w and w, are the same as those shown inH6. 12 respectively. The delay circuit 20 delays the output signal w, ofthe UN frequency divider 14 by a delay time T so that the output signalW8 of the differentiator 19 is inhibits in the inhibit gate 16 in caseof synchronization condition. If the output signal W7 is not obtained incase of no synchronization condition, the output pulse w passes throughthe inhibit gate 16 opened and triggers the monostable circuit 17 so asto reset the circuits 9, 10 and 14 through'a line 18.

More detailed discussion of an embodiment of this inven tion will bedescribed below with reference to FIGS. M, 15, 16 and 17. MG. 14 shows adetailed block diagram of an embodiment of this invention correspondingthe embodiment shown in FIG. s. In this block diagram, only differentcircuits or connections from the embodiment of FIG. 6 will be described.A shaper 22 generates a triangular wave W 4 synchronized with an inputsignal W13 by limiting the level of the input signal W3). Adifierentiator 23 generates a pulse train each pulse of which is timedwith the start of each pulse of the triangular wave. An unblankingamplifier 24 amplifies the output of the sweep gate 2 to apply theamplified signal to the grid or cathode of a cathode-ray tube through aterminal 25 to turn on the beam for the duration of the sawtooth wave w.The first gate 6 generates the gate signal w, started in response to thestart of the gate signal w and terminated in response to the terminationof the gate signal w, or when the level of the sawtooth wave reaches apredetermined threshold level. The monstable circuit 9 comprises aninhibit gate 9-1 and a rnonostable circuit 9-2. The counter comprises aring counter 10-1 and a bistable circuit 10-2. This bistable circuit10-2 is set by the trigger pulse generated from the counter 10-1 whenthe counting state of the counter 10-1 reaches a counting statecorresponding to the sharpest slope of the sawtooth wave w and reset bythe output of the inhibit gate 16. The 1/N frequency divider 14comprises a scale-of-N counter 14-1, a pulse converter generating anoutput pulse when the counting state of the counter 14-1 becomesindicate the highest counting state thereof, and a bistable circuit 14-3set by the output pulse of the pulse converter 14-2 and reset inresponse to the termination of the output (the wave W2) of the sweepgate 2). The rnonostable circuit 17 may be provided with a manual resetswitch as shown.

With reference to FIG. 15, the automatic selection operation ofdifferent sweep periods of the sawtooth wave W3 from longer to shorteris at first described. An input signal W13 is applied through a line 1to the shaper 22, in which this input signal W13 is converted to atriangular wave w,4. This triangular wave w,4 is converted to a pulsetrain w including pulses P P P P, which are applied to the sweep gate 2.The state of the output signal w of the sweep gate 2 is reversed to thestate 1 in response to the first pulse P In response to this reverseoperation, the sawtooth wave generator 4 starts to generate the sawtoothwave W3. The first gate 6 generates the output signal w. starting inresponse to the start of the signal W and terminating in response to thetermination of the signal w or when the level of the sawtooth wave wreaches a threshold level (predetermined in this first gate 6 to set thenumber of cycles of the input signal W13 included within the duration ofthe sawtooth wave W3). Any of pulses P P P;,,.... included within theduration of the signal w are passed through the AND gate 8 and theinhibit gate 9-1 since this inhibit gate 9-1 is not inhibited from theoutput of the monostable circuit 17 at this time. Pulses (i.e.; P and Pin this case) passed through the AND gate 8 and 9-1 trigger themonostable circuit 9-2, which generates a triangular wave w,2. Pulsestimed with the start instants of cycles of the triangular wave W12trigger the ring counter 10-1 and the hold circuit 21, so that thecounting state of the counter 10-1 changes successively to control thesawtooth wave generator 4 generate the sawtooth wave W3 W havingsuccessively sharper slopes, and so that the state of the hold circuit21 is reversed. In response to the reverse operation of the state of thehold circuit 21, the state of the output signal W2 of the sweep gate 2is restored from the state 1 to the state 0. A time A l is a delay timein the hold circuit 21 from a time triggered by the start of the wavew,2 to a time resetting the sweep gate 2. The above mentioned operationsare repeated until none of pulses P P P is included within the durationof gate pulse (W4). FIG. shows three cycles of the above-mentionedoperations. When the counting state of the counter 10-1 reaches acounting state corresponding to the sharpest slope of the sawtooth waveW3, the state of the bistable circuit 10-2 is set and this set state ofthe bistable circuit 10-2 resets the state of the rnonostable circuit4-2.

As mentioned above, synchronization between the input signal W13 and thesawtooth wave W3 is performed. Waves W13, W14, w W3, W2, W4, W W w 5 andw.6 show operational waveforms of respective parts designated in FIG. 14in the case of this synchronization condition. In this case, the scale Nof the counter 14-1 is three so that the repetition frequency of thepulse train W is counted down to one-third at the output of the pulseconverter 14-2. Details of the operation of this condition are omittedsince this operation will be understood with reference to the operationof the forementioned embodiments (particularly to the operation oftheembodiment shown in FIG. 6).

The operation of the embodiment shown in FIG. 14 in the case of changingthe frequency of the input signal w,3 from higher to lower will bedescribed below as illustrated by signals W13 and w,3a in FIG. 16. Thisis the resetting operation to the gentlest slope of the sawtooth waveWhen the input signal" W 3 changes its frequency as show by the wave w3a, the "waves W14 and W are also changes to waves 14a and wrespectively as shown. However, the waves W W W4 and W5 are notchangedQWhile the bistable circuit assumes the state 1, the AND gate 13is opened so that pulses of the pulse train W passes through the openedAND gate 13. However since the frequency of the input signal w of thiscase is lower than the frequency of the former input signal W only twopulses can pass through the AND gate 13 at an interval where the ANDgate is opened by the signal W5. Accordingly, the state of the counter14-1 is not changed to the state 1 and the bistable circuit 14-3 assumesthe state 0. Therefore, the inhibit gate 16 is not inhibited at thistime. If the reference level of the comparator 19 is determined as shownby a point B on the wave W3, the comparator 19 generates a pulse w whenthe level of the s sawtooth wave reaches this reference level in thecomparator 19. This pulse W is applied to the rnonostable circuit 17 andthe bistable circuit 10-2 through the inhibit gate 16 opened. At thistime, the state of the rnonostable circuit 17 changes to the state 1lasting a duration T as shown by a wave w The wave w inhibits theinhibit gate 9-1 within an interval equal to the duration T and resetsthe counting state of the counter 10-1 to a state corresponding to thegentlest slope of the sawtooth wave W3. Moreover, the wave w resets thestate of the hold circuit 21 to the original state. At the same time,the state of the bistable circuit 10-2 is reversed bythe output pulse ofthe counter 10-1 of this case and restored by the wave w passed throughthe opened inhibit gate 16 while the reset of the rnonostable circuit9-2 is temporarily stopped by the reversed output of the bistablecircuit 10-2.

The principle of the determination of the point A (the threshold levelof the first gate 6) and of the point B (-the reference level of thecomparator 19) will be described below with reference to FIG. 17. Theduration I of the wave w obtained from the first gate 6 is concerned indetermining the number of cycles of the input signal (W13) included inthe duration of the sawtooth wave W3. This number of cycles is equal tothe displayed cycles on the cathode-ray tube in case of applying thisinvention to a cathode ray oscilloscope. This number of cycles isinversely proportional to the time length of the duration t,. In case ofthe above-mentioned synchronization, a condition t t is essential, where1 is the period of the input signal W13.

The set interval of the bistable circuit l2a(i.e.; the opened intervalof the AND gate 13 is designated by a time t starting from thetermination of the wave w (i.e.; the point A) and terminating inresponse to the termination of the wave W2. Therefore, a condition 2 NT(if N is three as assumed above, t 3 T) is satisfied. A time t shows anallowance of time for determining the point B and terminated in responseto the termination of the wave w. This time i is essential to be smallerthan the period t of the input signal W The reason for this is asfollows. If four cycles (larger than the number N 3) of the input signalW is to be included in the duration of the sawtooth wave w it isunneccessary to switch the slope of the sawtooth wave to the gentlestone unless the input signal w less than three cycles is received withinthe duration of the sawtooth wave w If the'input signal w more than fourcycles is received within the duration of the sawtooth wave W3, theoutput wave w (obtained at the point B) is not necessary since this wavew cannot pass the inhibit gate 16 inhibited by the output of thebistable circuit 14-3.

I claim:

' 1. An automatic sweep oscillator for generating a sawtooth wavesynchronized with any of input pulses, comprising:

a sawtooth wave generator for generating a sawtooth wave synchronizedwith one of a plurality of predetermined different slopes",

a hold circuit generating an output when the sawtooth wave reaches apredetermined level;

a control circuit for determining the duration of the sawtooth wave bystarting the sawtooth wave in synchronism with one of input pulses andby terminating the sawtooth wave in synchronism with the output of thehold circuit;

an automatic selection circuit for switching successively the diflerentslopes of the sawtooth wave from a gentle one to a sharp one and forstopping the above-mentioned switching when any of the input pulses isnot received within a predetermined first interval starting insynchronism with the start of the sawtooth wave, and W a reset circuitfor controlling the automatic selection circuit so that the sawtoothwave generator generates a sawtooth wave having the gentlest slope whenthe number of the input pulses received within a predetermined secondinterval included within the duration of the sawtooth wave and longerthan the first interval is less than the predetermined number.

2. An automatic sweep oscillator, as in claim 1 in which thepredetermined second interval starts from the termination of the firstinterval.

3. An automatic sweep oscillator, as in claim 1 in which thepredetermined second interval starts from the start of the firstinterval.

1. An automatic sweep oscillator for generating a sawtooth wavesynchronized with any of input pulses, comprising: a sawtooth wavegenerator for generating a sawtooth wave synchronized with one of aplurality of predetermined different slopes; a hold circuit generatingan output when the sawtooth wave reaches a predetermined level; acontrol circuit for determining the duration of the sawtooth wave bystarting the sawtooth wave in synchronism with one of input pulses andby terminating the sawtooth wave in synchronism with the output of thehold circuit; an automatic selection circuit for switching successivelythe different slopes of the sawtooth wave from a gentle one to a sharpone and for stopping the above-mentioned switching when any of the inputpulses is not received within a predetermined first interval starting insynchronism with the start of the sawtooth wave, and a reset circuit forcontrolling the automatic selection circuit so that the sawtooth wavegenerator generates a sawtooth wave having the gentlest slope when thenumber of the input pulses received within a predetermined secondinterval included within the duration of the sawtooth wave and longerthan the first interval is less than the predetermined number.
 2. Anautomatic sweep oscillator, as in claim 1 in which the predeterminedsecond interval starts from the termination of the first interval.
 3. Anautomatic sweep oscillator, as in claim 1 in which the predeterminedsecond interval starts from the start of the first interval.